Chitin based heteroatom-doped porous carbon as electrode materials for supercapacitors

Carbohydrate Polymers 173 (2017) 321–329

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Chitin based heteroatom-doped porous carbon as electrode materials for supercapacitors Jie Zhou ∗ , Li Bao, Shengji Wu, Wei Yang, Hui Wang College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou, 310018, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 31 March 2017 Received in revised form 31 May 2017 Accepted 1 June 2017 Available online 2 June 2017 Keywords: Heteroatom-doped carbon Chitin biomass Porous structure Surface chemistry Supercapacitor

a b s t r a c t Chitin biomass has received much attention as an amino-functional polysaccharide precursor for synthesis of carbon materials. Rich nitrogen and oxygen dual-doped porous carbon derived from cicada slough (CS), a renewable biomass mainly composed of chitin, was synthesized and employed as electrode materials for electrochemical capacitors, for the first time ever. The cicada slough-derived carbon (CSC) was prepared by a facile process via pre-carbonization in air, followed by KOH activation. The weight ratio of KOH and char plays an important role in fabricating the microporous structure and tuning the surface chemistry of CSC. The obtained CSC had a large specific surface area (1243–2217 m2 g−1 ), fairly high oxygen content (28.95–33.78 at%) and moderate nitrogen content (1.47–4.35 at%). The electrochemical performance of the CS char and CSC as electrodes for capacitors was evaluated in a three-electrode cell configuration with 6 M KOH as the electrolyte. Electrochemical studies showed that the as-prepared CSC activated at the KOH-to-char weight ratio of 2 exhibited the highest specific capacitance (266.5 F g−1 at a current density of 0.5 A g−1 ) and excellent rate capability (196.2 F g−1 remained at 20 A g−1 ) and cycle durability. In addition, the CSC-2-based symmetrical device possessed the desirable energy density and power density of about 15.97 W h kg−1 and 5000 W kg−1 at 5 A g−1 , respectively. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Currently, developing environmentally friendly, sustainable and affordable energy resources is still a great challenge for global researchers. For achieving this goal, supercapacitors have been regarded as one of the most promising electrochemical energy sources for energy storage and conversion devices, because of their high power density and long cycle life (Simon & Gogotsi, 2008). It is anticipated to develop new cost-effective electrode materials with improved performance. Carbon-based materials are expected to deliver outstanding performance as the electrode material for supercapacitors, because of their advantageous properties such as light weight, fast charging/discharging rates and bipolar operational flexibility (He et al., 2013; Liu et al., 2015; Wahid, Puthusseri, & Ogale, 2014; Zhang & Zhao, 2009). Porous activated carbon materials, especially, are appealing materials for supercapacitors because of their low cost, extraordinary tunable structure and surface chemical properties, as well as chemical, mechanical and thermal stability (Elmouwahidi, Zapata-Benabithe,

∗ Corresponding author. E-mail address: [email protected] (J. Zhou). http://dx.doi.org/10.1016/j.carbpol.2017.06.004 0144-8617/© 2017 Elsevier Ltd. All rights reserved.

Carrasco-Marin, & Moreno-Castilla, 2012; Gao et al., 2015; Sudhan, Subramani, Karnan, Ilayaraja, & Sathish, 2017). Electrical double layer capacitance (EDLC) and pseudocapacitance are two types of capacitance based on different electrochemical energy storage mechanisms. In electrical doublelayer capacitors, ions diffuse and accumulate in the electrical double layer formed along the interface between the electrolyte and the electrode, and pseudocapacitance results from the reversible Faradic redox reactions of electroactive species on the electrode surface (Yang, Liu et al., 2016; Zhang et al., 2013). To date, heteroatom-doped porous carbons have attracted increased interest, owing to their potential scale of energy storage and conversion (Ling et al., 2016; Ramasahayam et al., 2015). Nitrogen and oxygen incorporation can not only improve the surface hydrophilicity and conductivity of carbon materials, but also contribute pseudocapacitance to promote electrochemical redox reactions (Milczarek, Ciszewski, & Stepniak, 2011). Diverse methodologies have been developed to introduce nitrogen or oxygen species into the carbon framework by using various substances such as seaweed, melamine, and acetonitrile as precursors, or through chemical vapor deposition (CVD) or quick KOH activation ˜ processes (Raymundo-Pinero, Leroux, & Béguin, 2006; Tomohiro, Masashi, Hideya, & Satoru, 2013; Yang, Yu et al., 2016; Zhang et al., 2008). Yet, given the great potential in the field of electrochemi-

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cal applications, it would be more significant to explore infusing sustainable biomass with a large amount of heteroatoms, as a precursor to synthesizing novel carbons for electrodes. Chitin is a type of nontoxic, abundant, naturally occurring polysaccharide. Its annual biosynthesis amount is about multibillion tons, which is almost same with the annual production of cellulose. Chitin consists of ␤-(1, 4)-linked-2-acetamido-2deoxy-d-glucopyranose units (GlcNAc), and contains about 6.9 wt% nitrogen from N-acetamido groups. Thus chitin has received much attention as an amino-functional biomass precursor for synthesis of nitrogen-doped carbon using simple pyrolysis (Nogi, Kurosaki, Yano, & Takano, 2010; Yan et al., 2015; Yang et al., 2012). To the best of our knowledge, there is few literatures discussing the preparation of nitrogen-containing carbonaceous materials from chitin biomass and their potential application in electrochemical capacitors (Chen, 2015; Qu et al., 2015). Chitin can be obtained from diverse biomass sources such as crustacean (crab, shrimp, and krill) shells, insects, and fungal mycelia. Insect cuticles may be the best choice, as they contain much less inorganic matter than crustacean shells. A cicada is an insect of the order homoptera, suborder auchenorrhyncha, in the superfamily cicadoidea. Cicada sloughs, also known as periostracum cicadae, are the shells that are shed when cicada larvae become imagoes. This material is considered a new alternative source for chitin (Sajomsang & Gonil, 2010). The recovery rate of chitin from cicada slough is much higher than that from rice-field crab shell. Hence, cicada slough is expected to be an ideal biomass candidate for the preparation of nitrogen-rich porous carbon. In this study, we report a new strategy to fabricate cicada sloughderived carbon (CSC-x) with desirable performance as the electrode material for electrochemical capacitors via combined precursor air carbonization and a KOH chemical activation process. CSCs offer the features of well-developed microporosity, large surface area, and abundant doped oxygen and nitrogen heteroatoms. The synergetic effects of microporous structure and heteroatoms (oxygen and nitrogen) incorporated into the carbon framework, on the capacitive behaviors of CSC-based electrodes, have been investigated comprehensively. 2. Experimental 2.1. Raw materials Cicada sloughs of C. atrata Fabricius were collected in the Xiasha campus of Hangzhou Dianzi University. Acetone, ethanol, hydrochloric acid (HCl), potassium hydroxide (KOH), and polytetrafluoroethylene (PTFE) latex (60 wt% dispersion in H2 O) were supplied by Sinopharm Chemical Regent Co. Ltd., China. All reagents were of analytical grade and used as received without further purification. Millipore water was used in all experiments. Nickel foam (99.9%, 1.5 cm in thickness), acetylene black and hydrophilic diaphragm (separator) were purchased from Hunan Corun New Energy Co., Ltd. The nickel foam was initially immersed in 0.1 M HCl solution for 24 h, and then rinsed with acetone and ethanol in an ultrasonic bath. Finally, it was dried in a vacuum oven at 50 ◦ C for 8 h. 2.2. Preparation of heteroatom-doped porous carbon derived from cicada slough Fig. 1 illustrates the synthesis procedure for cicada slough-derived carbon, including demineralization and air precarbonization followed by KOH activation. In a typical synthesis, 45 g of raw cicada slough were immersed in 1000 ml of 10 wt% HCl solution at room temperature for 24 h to remove inorganic materi-

als such as calcium carbonate and adhered mud. Subsequently, the cicada sloughs were washed with deionized water until neutrality was reached. They were then dried in a conventional oven at 60 ◦ C for 12 h. The resultants were mechanically pulverized in a grinder and passed through an 80-mesh screen. In order to remove the soft parts of the biopolymer in the cicada slough selectively, and boost the pores to obtain a porous scaffold, the cicada slough powder was pre-carbonized at 240 ◦ C for 3 h in air. The yield of char was approximately 0.60 g/g dry cicada slough. It has been reported that air carbonization is an efficient way to increase the activation yield of phenolic resin fiber (Worasuwannarak, Hatori, Nakagawa, & Miura, 2003). KOH activation is a conventional and effective methodology to significantly enhance the specific surface area and develop the micropores of carbonaceous materials (Zapata-Benabithe, Carrasco-Marín, & Moreno-Castilla, 2012). Thus, to enlarge the surface area and improve the limited porosity, the air-treated char from the cicada slough was activated by grinding homogenously with KOH slices as quickly as possible in an agate mortar at the various weight ratios (KOH: char) of 1:2, 1:1 2:1, and 3:1 followed by heating at 800 ◦ C for 1 h in N2 atmosphere with a ramp rate of 3 ◦ C min−1 . The resulting dark powder was then washed with 1 M HCl solution and distilled water until the pH of the filtrate became neutral. Finally, the carbons were dried in a vacuum at 50 ◦ C for 12 h. The obtained samples were denoted as CSC-x, where x represents the mass ratio of solid KOH to char. 2.3. Characterization of porous carbons Nitrogen adsorption/desorption isotherms of the cicada sloughderived carbons were measured at 77 K using an autosorb iQ instrument (Quantachrome U.S.). Before measurement, the carbon samples were degassed under vacuum at 180 ◦ C for at least 6 h. The specific surface area was calculated with the multi-point Brunauer-Emmett-Teller (BET) method. The total pore volume was estimated from the adsorbed amount at a relative pressure P/P0 of 0.995. The micropore surface area and micropore volume were obtained using the t-plot method. The pore size distribution was calculated using the Non-local Density Functional Theory (NLDFT) equilibrium model for slit-pore geometry based on the adsorption and desorption data. Surface morphology of the as-prepared carbons was observed through a thermal field emission scanning electron microscope (FESEM; HitachiSU-70, Hitachi Corp., Tokyo, Japan). X-ray diffraction (XRD) patterns were recorded on a powder diffractometer (Rigaku, MiniFlex600) with Ni-filtered Cu-K␣ radiation (␭ = 0.154 nm). X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the surface chemistry of the carbons on a Thermo Scientific Escalab 250Xi (U.S.) using a monochromatic Al-K␣ X-ray source. Wide-survey spectra were recorded, providing the surface elemental composition. C1 s hydrocarbon peak at a binding energy of 284.6 eV was used to calibrate the binding energies. 2.4. Electrochemical measurements The evaluation of electrochemical performances was carried out on a Zahner electrochemical workstation (German) at ambient conditions using a three-electrode configuration and a two-electrode symmetrical configuration in 6 M KOH electrolytes. The working electrode (1 cm2 ) was prepared using a well-mixed slurry consisting of 85 wt% active materials, 10 wt% of acetylene black, 5 wt% PTFE binder and a small quantity of alcohol. The homogeneous slurry was coated onto nickel foam substrate that served as a current collector. The assembled electrode was dried in a vacuum oven at 120 ◦ C for 10 h, and then pressed under a pressure of 10 MPa for 30 s. For

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Fig. 1. Schematic illustration of procedure for cicada slough-derived carbon synthesis.

three-electrode configuration, an Hg/HgO electrode and platinum foil were used as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GC) were conducted under a potential window of −1.0 to 0 V. The electrochemical impedance spectroscopy (EIS) measurements were recorded in a frequency range from 100 kHz to 0.01 Hz with an alternating current amplitude of 5 mV. The specific capacitance of the electrodes (C), in the unit of F g−1 , was calculated from the GC curve according to Eq. (1): I × t m × V

C=

(1)

where I is the discharge current (A); t is the discharge time (s); m is the mass of the active material in an electrode (g); and V is the potential window (V). For the two-electrode configuration, two symmetrical work electrodes, with identical or nearly identical amounts of active materials, were separated by a polypropylene membrane and evaluated with CV measurements at different scan rates—from 10 to 200 mV s−1 1—and GC tests at different current densities—from 0.1 to 5 A g−1 —within a potential window of 0–1.0 V. The specific capacitance of the symmetric electrodes (CS , F g−1 ), and the energy density (E, Wh kg−1 ) and power density (P, W kg−1 ) of the symmetrical electrode system, were determined using the following equations: CS =

2 × I × t m × V

(2)

E=

CS × V 2 2 × 3.6

(3)

P=

3600 × E t

(4)

3. Results and discussions 3.1. Structural characterization To better understand the porous structure of resultant carbons derived from cicada slough, N2 adsorption/desorption and pore size distribution were performed as shown in Fig. 2. The shape of all the

isotherms in Fig. 2(a) can be defined as a combined I/IV type according to IUPAC classification. The steep increase at relatively low pressure (P/P0 ), and the hysteresis loop observed in the isotherms, reveal the existence of abundant micropores and a small quantity of mesopores, respectively, that were created by KOH activation. The profiles of pore size distribution of carbon samples displayed in Fig. 2(b) are in line with the N2 adsorption/desorption analysis. It can be seen that the majority of pores in the CS char and CSC samples fell within the domain of micropores (pore width <2 nm). Cicada slough char might be activated with KOH at a high temperature of 800 ◦ C according to the following processes (He et al., 2012; Lillo-Ródenas, Cazorla-Amorós, & Linares-Solano, 2003): KOH + 2C → 2 K + 3H2 + 2 K2 CO3

(5)

KOH + C → 2 K2 O + 2 H2 + CO2

(6)

The void spaces in the skeleton of the cicada-slough char would be generated from the gas evolution and occupied by the resultant potassium compounds, such as K2 CO3 and K2 O. Washing with dilute HCl solution and distilled water could remove the compounds and boost the number of micropores effectively. Detailed structural properties and yields of all carbons are shown in Table 1. The CS char sample shows a low SBET of 125 m2 g−1 and a small total pore volume of 0.053 cm3 g−1 after air precarbonization at 240 ◦ C. And almost all of the pores in the CS char were micropores, which could be attributed to the demineralization and mild air etching. Both the specific surface area and the pore volume of the CSC samples increased significantly with increasing KOH-to-char ratio, from 0.5 to 3. The CSC-3 sample exhibited the highest SBET of 2217 m2 g−1 and the largest total pore volume of 1.091 cm3 g−1 after KOH activation at 800 ◦ C—both characteristics beneficial for high-performance energy storage. In addition to specific surface area and pore volume, the yield of CSC samples is another important factor to evaluate their production feasibility. The carbon yield decreases with increasing KOH to char ratio as shown in Table 1. Although the CSC-3 possesses the highest surface area of the studied samples, its carbon yield is only 10.9%. Considering both the carbon yield and surface area, CSC-2 sample had the appropriate yield of 31.1% and high specific surface area of 1745 m2 g−1 .

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Fig. 2. (a) Nitrogen adsorption/desorption isotherms and (b) corresponding pore size distribution curves using the NLDFT model of cicada slough-derived carbons.

Table 1 Structural properties and yields of carbon samples derived from cicada slough. Sample

SBET (m 2 g−1 )

Smicro (m2 g−1 )

Smeso (m2 g−1 )

Vmicro (cm3 g−1 )

Vtotal (cm3 g−1 )

Da (nm)

Yield (%)

CS char CSC-0.5 CSC-1 CSC-2 CSC-3

125 1243 1483 1745 2217

122 1165 1400 1660 2097

3 78 83 85 120

0.047 0.472 0.577 0.693 1.020

0.053 0.568 0.673 0.730 1.091

1.242 1.364 1.428 1.211 1.452

60.8 42.0 38.2 31.1 10.9

Da , average pore diameter.

Fig. 3. XRD patterns and SEM images of cicada slough-derived carbons. (a) XRD patterns; (b) The dense surface of cicada slough-derived char; (c) The surface of CSC-0.5 with harsh hollows; (d) The surface of the CSC-1 with smooth hollows; (e) The “bowl-like” model of the CSC-2 surface; (f) The surface of the CSC-3, which was etched intensively by the potassium hydroxide.

XRD patterns of CS carbons are shown in Fig. 3(a). These patterns display two diffraction peaks. The intense broad peak exhibited at 2␪ = 24◦ , corresponding to the (002) diffraction, implies a partly graphitized structure. Another weak peak at 2␪ = 43◦ can be attributed to the (101) diffraction, suggesting that the as-prepared CS carbon frameworks were amorphous. Increasing KOH-to-char ratio from 0.5 to 3, the intensity of the (002) diffraction peak of CSC dramatically reduces. The graphitic crystalline structures of

the CS char destroyed by the KOH chemical activation results in the change in XRD pattern. A significant increase in the low-angle scatter pattern from CSC versus CS char demonstrates the presence of dense pores, which is consistent with the N2 adsorption/desorption measurements and SEM observations. Fig. 3(b)–(f) show the SEM images of the CS char and carbon samples after KOH activation. SEM morphology of the resulting CS carbons reveals a gradual structural evolution as the KOH-to-char

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325

Fig. 4. (a) XPS survey of cicada slough-derived carbons; XPS deconvoluted spectra (b) N1 s and (c) O1 s for representative CS char and CSC-2.

ratio increases. It can be seen in Fig. 3(b) that the cicada slough derived char had the dense surface. The CSC-0.5 as shown in Fig. 3(c) exhibits harsh hollows like volcanic vent. The hollows on CSC-1 surface in Fig. 3(d) were smoother than those on CSC-0.5. And clearly, CSC-2 possessed an uneven surface and had smoother and deeper “bowl-like” hollows, as presented in Fig. 3 (e). A 3D architecture is formed by interconnected channels derived from the decomposition of chitin biomass. It also can be seen that the CSC-3 surface in Fig. 3(f) was etched intensively by the potassium hydroxide and covered by numerous small particles. Heteroatoms have been shown to improve the performance of carbon materials in the field of energy storage and conversion. XPS was employed to investigate the surface elemental compositions and configurations of heteroatoms in carbon materials and to further understand their roles in electrochemical capacitors. As shown in Fig. 4(a), CS char and CSC show distinct three peaks, of C1s, O1s and N1s. The surface elemental composition results from the XPS analysis for carbons are listed in Table 2. It can be seen from Table 2 that the contents of oxygen and nitrogen heteroatoms on the surface of CS char were 27.23 at% and 16.26 at%, respectively. It should be noted that the oxygen content of all the cicada sloughderived carbons was more than 27 at%. As far as we know, this ratio is much higher than that of other biomass-derived carbons (Qu et al., 2015; Selvamani, Ravikumar, Suryanarayanan, Velayutham, & Gopukumar, 2015). In addition, nitrogen content decreased significantly, and oxygen content increased slightly, as the weight ratio of KOH to char increased from 0.5 to 3, demonstrating that the nitrogen and oxygen functionalities on the surface of the CSC derived from the decomposition of nitrogen-containing polysaccharide in cicada slough are dependent on the weight ratio of KOH to char. The high-resolution deconvoluted spectra of N1s and O1s for CS char and CSC-2 are displayed in Fig. 4(b)–(c). The corresponding peak assignments of N1s and O1s are summarized in Table 2. The N1s spectra shown in Fig. 4(b) can be resolved into several peaks, centered at about 397.8, 399.4, 400.7 and 402.1 eV, which can be attributed to characteristic nitrogen functionalities of pyridinic-N (N-6), pyrrolic-N (N-5), quaternary-N (N-Q) and oxidized-N (N-X) (Lin et al., 2015). The N-X state was not found on the CS char. N-Q was the predominant nitrogen configuration on the CS char, and its composition was about 8.62 at%, which is higher than the sum of the basic N-6 and N-5 species. Compared with the N-Q and NX, the N-6 and N-5 became the dominant states on CSC after KOH activation. Two peaks, centered at about 532.2 and 533.5 eV, were observed in the O1s spectra presented in Fig. 4(c); these can be ascribed to quinone/carbonyl (C O) (O-1) and ether (C O) (O-2) (Silva, Freitas, Freire, Castro, & Figueiredo, 2002). The composition of O-1 functionality was close to that of O-2 functionality on the CS char, CSC-0.5 and CSC-1 samples. CSC-2 had the highest O-1 composition among the carbons. When the weight ratio of KOH and

char was increased from 2 to 3, the composition of O-1 dramatically decreased. It is interesting to note that the sum of the N-6 and O-1 species of CSC-2 was almost same as that of the CS char, which is higher than that of the other carbons.

3.2. Electrochemical performance CV, GC and EIS tests were performed to evaluate the electrochemical characteristics of the obtained cicada slough-derived carbons in 6 M KOH aqueous electrolyte at ambient temperature using a three-electrode configuration. Fig. 5(a) shows the CV curves of the resultant carbons at a scan rate of 20 mV s−1 . The CV curves of the carbons all exhibited a slightly distorted and quasi-rectangular shape, suggesting the formation of an electrical double layer and equivalent series resistance in 6 M KOH aqueous electrolyte. The area enclosed by the CV curves indicates capacitance increases in the following order: CS char < CSC-0.5 < CSC-3 < CSC-1 < CSC-2. Representative CV profiles of CSC-2 at various scan rates are shown in Fig. 5(b). Increasing the scan rate from 10 to 200 mV s−1 caused the shape of the curves for CSC-2 to become more distorted, indicating that the limited ion diffusion and confined electrical conductivity at a high sweep rate resulted in uncompensated resistance. Moreover, the heteroatoms nitrogen and oxygen introduced extra pseudocapacitance into the carbon electrode system, through Faradic reactions. However, electrical charges have insufficient time to transfer within the micropores of CSC-2 when the voltammetric current is high, resulting in insufficient utilization. Galvanostatic charge/discharge behaviors of the carbons were investigated at current densities of 0.5–20 A g−1 . Fig. 5(c) shows the representative galvanostatic charge/discharge curves of CSC2 at different current densities. With increasing current density, the curves still retained their symmetric triangular shape, implying that CSC-2 possesses almost ideal EDLC behavior and the desirable electrochemical reversibility. There was also an insignificant voltage drop at the current switches in the galvanostatic charge/discharge curves, as presented in Fig. 5(c), suggesting that the CSC-2-based electrode had a small internal resistance. The specific capacitance values of carbons determined from galvanostatic discharge curves are shown in Fig. 5(d), among which CSC-2 exhibited the highest specific capacitance of 266.5 F g−1 at low current densities of 0.5 A g−1 . Although the specific area of CSC-2 was lower than that of CSC-3, the content of pyridinic-N, pyrrolic-N and quinone/carbonyl (C O) species on CSC-2 was higher than those on CSC-3. What’s more, when the discharge current density increased to 20 A g−1 , the CSC-2 still retained high specific capacitance of 196.2 F g−1 , indicating a high rate performance of the CSC-2 electrode. It is interesting to note that the CS char possessed a high specific capacitance of 120.5 F g−1 at a current density of 0.5 A g−1 in spite of its lowest specific surface area and enclosed area in the CV

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Table 2 Elemental compositions and peak assignment of N1 s and O1 s for the carbon samples from XPS analysis. Sample

CS char CSC-0.5 CSC-1 CSC-2 CSC-3

Surface elemental analysis (at%)

Peak assignment and composition (at%)

C

N

O

N-6

N-5

N-Q

N-X

O-1

O-2

56.51 66.70 67.46 67.98 64.75

16.26 4.35 2.88 2.24 1.47

27.23 28.95 29.66 29.78 33.78

3.32 2.01 1.46 1.06 0.69

4.33 1.25 0.46 0.29 0.24

8.62 0.92 0.52 0.60 0.29

/ 0.17 0.44 0.29 0.25

14.84 14.41 15.61 17.20 9.85

12.39 14.54 14.05 12.58 23.93

Fig. 5. (a) CV curves of carbons at a scan rate of 20 mV s−1 in a 6 M KOH aqueous solution; (b) representative CV curves of CSC-2 at scan rates of 10, 20, 50, 100, and 200 mV s−1 ; (c) representative galvanostatic charge/discharge curves of CSC-2 at different current densities; (d) calculated specific capacitance versus current density of carbons; (e) representative cycle stability of CSC-2 at a constant current density of 2 A g−1 ; (f) Nyquist plots of carbons. Inset is the enlarged view of the Nyquist plots of AC impedance.

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327

Fig. 6. Electrochemical behavior of CSC-2 in symmetrical two-electrode cell in a 6 M KOH aqueous solution (a) CV curves at scan rates of 10, 20, 50, 100, and 200 mV s−1 ; (b) galvanostatic charge/discharge curves at current densities of 0.4, 0.6, 1, 2, 3 and 5 A g−1 ; (c) Ragone plot; (d) Nyquist plot; inset is the enlarged view of the high-frequency region.

curve. It is believed that the heteroatoms on the CS char also played an important role in the GC properties. The specific capacitance of CSC-2 was much higher than the reported specific capacitance of shrimp shell-derived carbon (175 F g−1 ) at a current density of 0.5 A g−1 in the 6 M KOH solution (Qu et al., 2015). The superior specific capacitance observed in CSC-2 can be ascribed to the synergetic effects of large specific surface area and high content of pyridinic-N, pyrrolic-N and quinone/carbonyl (C O) species on the CSC-2 surface in-situ derived from cicada slough precursor, which could intensify the electrostatic adsorption of electrolyte ions for energy storage in the EDLC, and offer a reversible Faradaic reaction for pseudocapacitance, respectively. Pyridinic-N, pyrrolic-N and quinone/carbonyl (C O) species on the graphitic edge defects of the carbon matrix may participate in surface Faradic reactions and enhance the pseudocapacitance in alkaline electrolytes by the following electrochemical processes (Xu, Hou, Cao, Wu, & Yang, 2012) on the carbon surface: C ∗ − NH2 + 2OH− ↔ C ∗ − NHOH + H2 O + 2e− −

C ∗ H − NH2 + 2OH ↔ C∗ = NH + 2H2 O + 2e C∗ = O + 2OH− ↔ O − C ∗ − O + H2 O + 2e−

−

(3) (4) (5)

where C* is on behalf of the graphitic framework. A repetitive galvanostatic charge/discharge measurement at a current density of 2 A g−1 over 5000 cycles was conducted to investigate the impact of high oxygen content on the durability of the as-made CSC-2 electrode during the long cycles. The CSC-2 showed a specific capacitance of 232 F g−1 at the first cycle and 215 F g−1 at the 5000th cycle, as depicted in Fig. 5(e). This

means that CSC-2 retained almost 92.7% of the initial capacitance after 5000 charge/discharge cycles. The result indicated that pseudocapacitance introduced by enriched oxygen through Faradic charge-transfer reactions is stable, which is in agreement with pre˜ vious studies (Milczarek et al., 2011; Raymundo-Pinero et al., 2006; Zhang et al., 2008). It is suggested that CSC-2 could be a promising electrode-material candidate for supercapacitors because of its high capacitance, good rate performance and satisfactory cycling stability. EIS measurement was used to further investigate the resistive behavior of the cicada slough-derived carbon electrodes for supercapacitors. The Nyquist plot representing the real part (Z ) and imaginary part (Z ) of impedance was used to analyze the EIS data. The Nyquist plots in Fig. 5(f) exhibit similar curves. Three distinct parts can be observed in the Nyquist plots: a semicircle at high frequencies, a slash (about 45◦ ) and a nearly vertical line at low frequencies. The inset in Fig. 5(f) is the enlarged view of the Nyquist plots of AC impedance. The intercept and diameter of the semicircle on the real axis (Z ) at the high-frequency region can reflect the solution resistance (Rs ) and the charge transfer resistance (Rct ) respectively. It was observed that the Rs of the CS char was about 0.62 , which is almost twice that of the Rs of CSC after activation. It was supposed that Rs was influenced by the electrolyte resistance, the intrinsic and interfacial resistance of the current collector, and carbon (Kim & Yang, 2003; Meher & Rao, 2012). Obviously, the CS char electrode had high intrinsic and interfacial resistance in KOH electrolyte because of its poorly developed porosity. Rct decreased in the following order, similar to Rs : CS char > CSC-0.5 > CSC–1 > CSC–3 > CSC-2. High specific sur-

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face area and electrochemically active functionalities can intensify the charge transfer within the microporous CSC-2 electrode and result in low Rct values. At the low-frequency region, the slash was attributed to Warburg impedance. The intercept of the steep line on the real axis (Z ) at the low-frequency region can reflect the equivalent series resistance (ESR) of carbons. The order of ESR was same as that of Rct . In short, the impressive electrochemical performance of the CSC-2 electrode was strongly related to its structure and surface chemical characteristics, including the large micropore volume, high specific surface area, and plentiful oxygen and nitrogen heteroatoms. The excellent capacitive performances of the CSC-2-based electrodes were further investigated in a two-electrode cell system. As shown in Fig. 6(a), the CSC-2-based symmetrical device still displayed characteristic capacitive behavior with quasirectangular-shaped CV curves even at a high sweep rate of 200 mV s−1 , which indicated that the charge transfer rate within CSC-2 was rapid and effective. The GC curves in Fig. 6(b) exhibit similar symmetrical triangular shapes, and the CSC-2 electrode showed superior performance with a high specific capacitance of 153.6 F g−1 at a low current density of 0.4 A g−1 , while retaining a high value of 115 F g−1 at a high current density of 5 A g−1 . It can be observed from the Ragone plot for the CSC-2 electrode shown in Fig. 6(c) that the energy density was 21.33 Wh kg−1 at a current density of 0.4 A g−1 in 6 M KOH electrolyte. When the current density increased to 5 A g−1 , the energy density and power density were about 15.97 Wh kg−1 and 5000 W kg−1 , respectively. Higher energy density and power density can be obtained in a wider work voltage window (Qu et al., 2015). The Nyquist plot in Fig. 6(d) shows that the symmetrical CSC-2-based device had not only low solution, charge transfer and equivalent series resistance, but also a negligible Warburg impedance. It is suggested that CSC-2 electrodes are promising for use as industrial energy storage devices with desirable electrochemical performance. 4. Conclusions In summary, a facile synthesis approach was demonstrated to prepare novel heteroatom dual-doped carbons from cicada slough via mild air pre-carbonization and KOH activation. The resultant CSC-2 possessed well-developed micro porosity (0.73 cm3 g−1 ), high specific surface area (1745 m2 g−1 ), and rich heteroatoms oxygen and nitrogen. Its unique properties of microporous structure and surface chemical composition (especially pyridinic-N, pyrrolicN and quinone/carbonyl (C O) species) had synergistic effects on its superior capacitive behavior. The CSC-2 electrode exhibited high specific capacitance and good rate performance in a three-electrode cell configuration and desirable energy density and power density in a symmetrical two-electrode system with 6 M KOH as the electrolyte. Acknowledgements The authors are grateful for financial support from the National Natural Science Foundation of China (Project No. 21406044) and the Zhejiang Province Nature Science Foundation of China (Grant No. LQ17B060006). References Chen, T. (2015). CO2 adsorption on crab shell derived activated carbons: contribution of micropores and nitrogen-containing groups. Rsc Advances, 5(60), 48323–48330. Elmouwahidi, A., Zapata-Benabithe, Z., Carrasco-Marin, F., & Moreno-Castilla, C. (2012). Activated carbons from KOH-activation of argan (Argania spinosa) seed shells as supercapacitor electrodes. Bioresource Technology, 111, 185–190.

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